WO2021064662A1

WO2021064662A1 - Method and plant for activating catalysts

Info

Publication number: WO2021064662A1
Application number: PCT/IB2020/059249
Authority: WO
Inventors: Luca Toncelli; Stefano ZEGGIO; Fabio Bassetto; Michele CASARIN; Vito Di Noto; Enrico Negro; Angeloclaudio NALE; Yannick Herve BANG; Keti VEZZU'; Gioele PAGOT
Original assignee: Breton S.P.A.
Priority date: 2019-10-04
Filing date: 2020-10-02
Publication date: 2021-04-08

Abstract

The present invention refers to a method of activating a solid catalyst material, an activated catalyst obtainable by said activation method, a fuel cell, an electrolyzer, a metal-air battery, or a catalytic converter containing said activated solid catalyst, as well as a plant for carrying out said activation method.

Description

Title

METHOD AND PLANT FOR ACTIVATING CATALYSTS

DESCRIPTION

Field of the Invention

The present invention refers to method of activating a solid catalyst material, an activated catalyst obtainable by said activation method, a fuel cell, an electrolyzer, a metal-air battery, or a catalytic converter containing an activated solid catalyst, as well as a plant for carrying out said activation method.

Within the scope of the present invention and in the subsequent claims, the term catalyst is intended to mean a solid substance capable of promoting a certain chemical process by increasing the rate by which the products of such a chemical process are obtained.

Such ‘catalysts’ can implement their function: (i) by exchanging electrons with an external electrical circuit (in this case, the materials in question are known as ‘electrocatalyst materials’, EC, and are typically good electron conductors); or (ii) without exchanging electrons with an external electrical circuit (in this case, the materials in question are known as ‘catalyst materials’ and are not necessarily good electron conductors).

In particular, an electrocatalyst material, or more simply an electrocatalyst, is a solid material capable of reducing the overvoltage associated with a certain oxidation or reduction process {“redox process”). Such process involves one or more chemical species that are typically contained in a fluid phase (liquid or gas) that is contacted with the electrocatalyst. The reduction of the overvoltage associated with a certain redox process is crucial from a practical point of view, as it allows to maximize the energy conversion efficiency associated with the process itself.

In many cases, electrocatalysts are solid composites or nanocomposites comprising the following key components:

1 . a first component, on whose surface “active sites” apt to promote the redox process of interest are located; the performance of these active sites depends on their chemical composition and structure.

2. a second component, typically referred to as a “support”, which serves to put the first component (and therefore its “active sites’) in electrical contact with the electrical circuit outside the electrocatalyst. The support also plays a crucial role in defining the morphology of the electrocatalyst, making the transport of reagents and products of the redox process of interest close to the active sites as easy as possible. The development of composite or nanocomposite electrocatalysts is justified by taking into account that:

1 . In some cases, the electrocatalyst component on whose surface the active sites are located is extremely expensive (for example, it is based on metals of the platinum group). Electrocatalysts are therefore made comprising spherical carbon nanoparticles (typically characterized by a diameter of 30-50 nm) coated with pure platinum nanoparticles having a diameter of 3 nm [Applied Catalysis B: Environmental 56 (2005) 9-35]. The resulting nanocomposite electrocatalysts typically comprise only one-tenth of the platinum needed to make solid platinum electrocatalysts having the same performance.

2. In other cases, the electrocatalyst component on whose surface the active sites are located is a poor conductor of electric current (for example, it is based on metal oxides). For example, there are many electrocatalysts for the oxygen evolution reaction in an alkaline environment based on mixed iron, cobalt, and nickel oxides [Catal. Sci. Technol., 2014, 4, 3800-3821 ].

Such systems comprise oxide particles having nanometric dimensions supported on conductive materials, such as carbon or transition metals (for example nickel) [Adv. Energy Mater. 2016, 6, 1600621 ]. In this way, the length of the path that electrons should follow within the low conductive phases is minimized, thus avoiding the introduction of ohmic drops that reduce the efficiency of the redox process of interest. The “oxygen reduction reaction” (ORR) is an electrochemical process of considerable application interest as it is involved in the operation of many latest generation electrochemical energy-conversion devices, such as fuel cells (FC) [Chemical Reviews 2016 116 (6), 3594-3657\ and metal-air batteries (MAB) [Chem. Soc. Rev., 2012, 41, 2172-2192 ]. ORR is characterized by a very slow kinetics, leading to very significant overvoltages (of the order of many hundreds of millivolts at operating temperatures below about 250 °C). This overvoltage significantly reduces the energy conversion efficiency of the device using ORR.

Specifically, this problem is particularly relevant in the case of FCs with acid polymeric membranes (“proton exchange membrane fuel cells”, PEMFCs). Such systems typically operate at temperatures below 100 °C; in addition, they are easy to build, have very high energy conversion efficiencies (of the order of 60%) and are characterized by a high power density (of the order of 1 kW/kg). All these features make PEMFCs particularly suitable for powering: (i) small stationary electricity generators; and (ii) light vehicles such as cars. PEMFCs work by electrochemically reacting hydrogen and oxygen. In particular, hydrogen undergoes an electro oxidation process at the anode of the PEMFC, producing H⁺ ions. The latter migrate through the proton exchange membrane, where they recombine with the products of the ORR occurring at the cathode of the PEMFC; the circuit is closed by electrons migrating between the anode and the cathode. The only product of the process is water. Therefore, the operation of PEMFC does not generate greenhouse gases, such as carbon dioxide, which is instead produced in large amounts during the operation of conventional engines powered by fossil fuels. Ultimately, a wide spread of PEMFCs could certainly represent a solution to reduce greenhouse gas emissions and contribute to the containment of global warming.

ORR represents the “slow” stage of the process exploited in the operation of PEMFCs, as it introduces higher overvoltages. Therefore, it is necessary to obtain ORR electrocatalysts that are efficient, durable, and economical for a PEMFC to operate optimally. The latter point is particularly problematic as the best ORR electrocatalyst in the highly acidic environment that can be present at the PEMFC cathode is platinum. To date, the ORR electrocatalysts exhibiting the best compromise among performance, operating life and reduced platinum loading are nanocomposite materials made as described above (i.e., comprising carbon nanoparticles with a diameter of 30-50 nm coated with pure platinum nanoparticles having a diameter of about 3 nm). These electrocatalysts lead to overvoltages of the order of 400-450 mV. Thanks to the use of these electrocatalysts, it is possible to make PEMFCs capable of delivering 5-8 kW per gram of platinum introduced into the device. It should be noted that PEMFCs typically use platinum-based electrocatalysts both on the anode and the cathode. However, since the hydrogen electro-oxidation process in the presence of platinum electrocatalysts is enormously faster than the ORR, a very small amount of platinum is enough to obtain very low anode overvoltages, of the order of a few tens of mV. Therefore, the great majority of the platinum introduced into a PEMFC is located in the electrocatalyst used at the cathode. Modern electrocatalysts for the ORR of the state-of-the-art described above have many limitations, which include above all: (i) a high platinum loading to achieve the desired level of performance; and (ii) weak interactions between the support and platinum particles, that reduce the stability of the material under operating conditions and, as a result, its durability. There are many approaches to obtain electrocatalyst materials for the ORR with improved performance and durability. In particular, the International Application WO2017/055981 describes a preparation approach leading to carbonitride-based electrocatalysts having a core-shell morphology. These electrocatalysts consist of a “core” of electrical conducting material with a high surface area, coated by a carbonitride-based “shelf’ supporting nanoparticles of metal alloys. The active sites are located on the surface of the latter nanoparticles. The electrocatalysts just described have ORR performances and durability much higher than the state-of-the-art nanocomposite electrocatalysts for the following reasons:

1 . The active sites of core-shell electrocatalysts include, in addition to platinum (acting as an “active metal’), one or more “co-catalysts” (for example, nickel and copper). In this way the performance of the active sites increases considerably, and the overvoltage is lowered down to 50-70mV, as compared to pure platinum.

2. The carbonitride “shell” forms strong bonds both with the nanoparticles of metal alloys on which the active sites are located, and with the “core”. In this way, the morphology and chemical composition of the electrocatalysts are stabilized, thus considerably increasing the durability of the material as a whole.

The core-shell electrocatalyst materials described in WO2017/055981 are produced by a multistage process comprising the following main steps: (i) preparation of a core-shell precursor comprising a support consisting of conductive particles on whose surface suitable molecular or macromolecular systems are distributed; (ii) application of a multistage heat treatment to the core-shell precursor to obtain a core shell electrocatalyst; (iii) activation of the electrocatalyst thus obtained.

The activated electrocatalyst (EC-A) typically has very different and better performances in promoting the redox process of interest when compared to the raw electrocatalyst (EC-G). This occurs because the activation treatment allows the chemical composition, morphology and structure of the EC-G to be modulated deeply and in a controlled manner, with particular reference to the metal alloy nanoparticles on whose surface the active sites are located. In conclusion, the activation treatment plays an absolutely crucial role in obtaining electrocatalyst materials with improved performance and durability compared to the state-of-the-art.

The preparation procedure just discussed allows to easily obtain large amounts of EC-G, of the order of many grams per batch, already on a research laboratory scale. The industrialization process of EC-G production is also very simple and linear, involving exclusively an aqueous phase synthesis and vacuum heat treatments.

To date, however, the setups used to carry out the activation treatments necessary to obtain EC-A are able to produce only very limited amounts of product, of less than 100 mg per batch. Furthermore, such setups often cannot be easily applied on an industrial scale since this would introduce disrupting factors (for example, ohmic resistances in two-dimensional electrodes of conductive material in contact with the external circuit only on one side) difficult to quantify and often variable between batch and batch. This is a limitation that, in principle, could compromise the possibility of adopting the core-shell electrocatalysts described in practical applications in WO2017/055981 . As an example, even a single small 5 kW PEMFC generator for domestic use requires the use of 2-10 g of electrocatalyst. Therefore, while only 1 -2 batches of EC-G are enough to cover this need, in principle it would be necessary to carry out 20-100 activation treatments (each lasting several hours) to be able to obtain enough EC-A to be used for this home application.

DEFINITIONS

Unless otherwise defined, all terms of the art, notations, and other scientific terms used herein are intended to have the meanings commonly understood by those skilled in the art to which this description belongs. In some cases, terms with meanings that are commonly understood are defined herein for clarity and/or ready reference; therefore, the inclusion of such definitions in the present description should not be construed as being representative of a substantial difference with respect to what is generally understood in the art.

The terms “approximately” and “about” used in the text refer to the range of the experimental error that is inherent in the execution of an experimental measurement. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (/.e., meaning “comprising, but not limited to”), and are to be considered as a support also for terms such as “consist essentially of”, “consisting essentially of”, “consist of”, or “consisting of”.

The terms “consists essentially of”, “consisting essentially of” are to be construed as semi-closed terms, meaning that no other ingredients affecting the novel features of the invention are included (optional excipients may therefore be included).

The terms “consists of”, “consisting of” are to be construed as closed terms.

Summary of the Invention

The activation method according to the present invention is an experimental setup by which it is possible to increase the amount of EC-G that can be activated in a single treatment, to obtain the final EC-A electrocatalyst material. Also on a laboratory scale, the activation method described herein allows to obtain EC-A batches having a mass equal to many grams, thus improving the mass obtainable by almost two orders of magnitude according to the procedures described in WO2017/055981. Therefore, the present invention relates, in a first aspect thereof, to an activation method according to claim 1 ; preferred features of the method are reported in the dependent claims.

More particularly, the method of activating an initial solid catalyst material according to the invention comprises a first step and a second step in sequence, each of which comprises the treatment of said solid material with a liquid containing ionic species, wherein an electric current and/or an electrochemical potential is applied in said first or second step, and at least a subsequent separation step of separating the activated solid material from the liquid.

The Applicant has found that, thanks to the aforementioned specific features of the activation method according to the invention, it is possible to achieve a series of very advantageous technical effects, including: activating consistent amounts of solid catalyst materials, typically supplied in the form of powders, even in a single treatment; modulating the flow of electric current flowing through the material and/or the electrochemical potential applied to the material itself; selectively removing metals other than: (i) platinum; (ii) other metals of the platinum group, such as palladium, ruthenium, rhodium, iridium, osmium originally present in the catalyst.

In a preferred embodiment, said first and/or second steps are carried out at least once, preferably carried out repeatedly 1 to 3 times, and optionally a separation step of separating the activated solid material from the liquid is carried out downstream of said first and/or second steps.

Preferably, the initial solid material is in the form of powders.

In the method according to the invention, an electrocatalyst can be used as a catalyst, preferably a carbonitride-based electrocatalyst having a core-shell morphology, a catalyst based on metallic elements of the platinum group, preferably palladium, platinum and/or rhodium, or a catalyst comprising graphene oxide, graphene nitride, graphene, or graphene functionalized with -COOH and/or -OH groups.

Examples of electrocatalysts that may be used in the present invention are those carbonitride-based and having a core-shell morphology described for example in WO20 17/055981.

Examples of catalysts are those based on metallic elements of the platinum group (Platinum Group Metals, PGM), such as palladium, platinum and/or rhodium for applications in catalytic converters.

Such catalysts are capable of promoting the oxidation reaction of pollutants, such as CO, hydrocarbons (HC) and nitrogen oxides (generally indicated as NOx and comprising about 90% NO and about 10% N0₂) to N0₂, and/or C0₂ and/or H₂0. Examples of catalysts comprising graphene oxide, graphene nitride, graphene, or graphene functionalized with -COOH and/or -OH groups are those described in WO2018/122368.

Examples of liquids containing ionic species that may be used in the present invention are selected from the group comprising water, alcohols, organic acids, organic carbonates, ionic liquids, and a mixture thereof.

Preferably, the liquid containing ionic species is a solution comprising: (i) at least one soluble ionic compound, preferably selected from NaCI, KBr, NaF, Csl, NaN0₃, K₂S0₄, or a mixture thereof; (ii) at least one acid, preferably selected from HCI, HNO₃, HCIO₄, H₂SO₄, H₃PO₄, CH₃COOH, or a mixture thereof; (iii) or at least a NaOH, KOH, Mg(OH)₂, NH₃ base, or a mixture thereof.

Preferably, the above solution has a pH from -2 to 16, more preferably from 0 to 3, or from 11 to 14.

Optionally, in said first and/or second steps a gas, preferably a gas selected from nitrogen, argon, neon, helium, krypton, xenon, oxygen, hydrogen, carbon dioxide, or a mixture thereof, is bubbled through the liquid.

Preferably, the first and/or second steps are carried out by means of stirring and/or heating.

Preferably, the first and/or second steps are carried out at a temperature comprised between -200 °C and 500 °C for a time comprised between 0.01 and 600 hours, preferably between about 20 °C and 75 °C for a time comprised between about 0.1 and 48 hours.

In a preferred embodiment, the electrochemical potential is lower than 1 .5 V vs. RHE, preferably between 0.2 and 0.9 V vs. RHE.

In a further preferred embodiment of the method according to the invention, the electric current is comprised between 0.00001 and 5000 mA, preferably 50 mA. Preferably, the first step is carried out in a first vessel and said second step is carried out in a second vessel, wherein the liquid containing ionic species is transferred from the first to the second vessel or vice versa.

In a further embodiment of the method according to the invention, the step of applying an electric current and/or an electrochemical potential comprises a) a first sub-step of entering the liquid into a first vessel containing a reference electrode and b) a second sub-step of transferring the liquid from said first vessel to a second vessel containing a working electrode.

Preferably, the step of separating the activated solid material from the liquid is obtained by means of filtration, centrifugation and/or flotation.

In accordance with a second aspect thereof, the present invention relates to an activated solid catalyst material obtainable from the activation method as described above, said initial solid material preferably being an electrocatalyst, more preferably a carbonitride-based electrocatalyst having a core-shell morphology, a catalyst based on metallic elements of the platinum group, more preferably palladium, platinum and/or rhodium, or a catalyst comprising graphene oxide, graphene nitride, graphene, or graphene functionalized with -COOH and/or -OH groups.

According to a third aspect thereof, the present invention relates to a fuel cell, an electrolyser, a metal-air battery, or a catalytic converter containing a solid catalyst activated as described above.

In accordance with a fourth aspect thereof, the present invention relates to a plant for carrying out the activation method as described above, comprising a first apparatus and a second apparatus containing a liquid containing ionic species and sequentially connected for transferring the liquid from the first to the second apparatus or vice versa, wherein a working electrode is present in said first or second apparatus. Preferably, in the plant according to the invention, the apparatus containing the working electrode is an electrochemical reactor, more preferably said reactor comprises a first vessel containing a reference electrode and a second vessel for transferring the liquid which contains a working electrode.

Brief Description of the Figures

Further features and advantages of the invention will become clearer from the following description of some of the preferred embodiments thereof, provided below for illustrative and non-limiting purposes, with reference to the attached drawings. In such drawings:

- Figure 1 is a block diagram of a plant for the activation of the solid catalyst according to the method of the present invention;

- Figure 2 is a schematic view of a plant for the activation of the solid catalyst according to a preferred embodiment of the present invention;

- Figure 3 shows the parameters of the activation treatment of the PtNM-G electrocatalyst (a) Intensity of the oxidative current flowing through the reactor; (b) ohmic resistance at the reactor terminals; (c) potential at the terminals (includes correction for ohmic drop);

- Figure 4 shows the CV-TF-RRDE (Cyclic Voltammetry Thin-Film Rotating Ring- Disk Electrode) profiles in the ORR of the reference electrocatalyst “Pt/C ref.” and of PtNM-A;

- Figure 5 shows the parameters of the activation treatment of the PtCu1-G electrocatalyst (a) Intensity of the oxidative current flowing through the reactor; (b) ohmic resistance at the reactor terminals; (c) potential at the terminals (includes correction for ohmic drop);

- Figure 6 shows the CV-TF-RRDE profiles in the ORR of the reference electrocatalyst “Pt/C ref.” and of PtCu1-A;

- Figure 7 shows the CV-TF-RRDE profiles in the ORR of the reference electrocatalyst “Pt/C ref.” and of PtCu2-A;

- Figure 8 shows the CV-TF-RRDE profiles in the ORR of the reference electrocatalyst “Pt/C ref. 2” and of PtCu3-A. Detailed Description of the Invention

In the following description, identical reference numbers are used to indicate the same elements or products for illustrating the figures.

Instead, literal references are also used to subsequently describe these elements or products in detail.

Different features in the individual embodiments illustrated in the description can be combined with each other at will, should one make use of the advantages resulting specifically from a particular combination.

With reference to FIGURE 1, the method according to the invention is carried out with the aid of an implant 300 consisting of a first apparatus 100 and a second apparatus 200, said apparatuses 100 and 200 contain a liquid containing ionic species (not shown in the figure) and are connected in sequence, wherein a working electrode 101 is present in said first apparatus 100 or second apparatus 200. Preferably, the transfer of the liquid from the first apparatus 100 to the second apparatus 200 takes place by means of a pump 102 which sucks the liquid from the first apparatus 100 to the second apparatus 200 or vice versa.

The catalyst material to be activated 103 is, for example, loaded into the first apparatus 100, activated, then moved into the second apparatus 200, activated, released from the second apparatus 200, and discharged 104 from the system 300. The apparatus 100 or 200 containing the working electrode 101 is an electrochemical reactor, preferably said reactor comprises a first vessel containing a reference electrode and a second liquid inlet vessel containing a working electrode (not shown in the figure).

A preferred embodiment of the plant 300 to carry out the activation method of the present invention is schematized in FIGURE 2; the operation of the system is described in detail below.

The first apparatus 100 consists of a vessel (A2) containing the liquid comprising the desired ionic species 1 (A1). Optionally, a suitable gas/vapor 3 (A3) may be bubbled through the liquid 1 (A1).

In a first step, STEP I, the solid catalyst material to be activated (MAT, not represented in vessel 2) is introduced in liquid 1 (A1) inside vessel 2 (A2), thus forming a dispersion.

The liquid 1 , comprising ionic species that can be used in the present invention, is the one indicated and described above.

The catalyst material to be activated MAT can be a carbon itride-based electrocatalyst having a core-shell morphology, or a catalyst based on metallic elements of the platinum group, or a catalyst comprising graphene oxide or derivatives thereof, as described above.

Optionally, such dispersion comprising the liquid A1 and MAT can be subjected to stirring and/or heating according to a suitable program, even a multistage one, for a suitable time. At the end of such a process, the dispersion is extracted from vessel 2 (A2) and MAT is separated from liquid 1 (A1) (for example, by means of filtration, centrifugation and/or flotation).

At this point, the process described above can be repeated as many times as desired, for example changing the liquid 1 (A1) or modifying experimental parameters such as the heating program, the stirring details and/or the gas/vapor 3 (A3) bubbled through the suspension comprising A1 and MAT.

In a subsequent step, STEP II, the solid catalyst material, to be activated or activated by the previous STEP I, 4 (MAT) is loaded into the second apparatus 200, i.e. a reactor 200 (C) that may optionally be thermostated to reach the desired temperature. Preferably, a pump 5 (B1) sucks the liquid 1 (A1) from vessel 2 (A2) and the liquid is conveyed into reactor 200 (C). Liquid 1 (A1) is conveyed inside a first vessel 6 (C1) of the second apparatus 200 inside which a reference electrode 7 (C2) is inserted, whose exact nature depends on liquid 1 (A1). This liquid is then used to wet the material to be activated 4 (MAT). MAT is positioned on a block of a good electric current conducting material that does not easily lead to redox processes 8 (C3). This material typically consists of graphite, gold, or platinum. Liquid 1 (A1) is filtered and pushed by the pump 5 (B1) through a porous septum 9 (C4). The porous septum 9 (C4) is an electronic insulator, typically made of filter paper and/or glass fiber. At this point, the liquid 1 (A1) is made to pass through a porous material and a good conductor of electric current 10 (C5); typically, this material consists of carbon fabric, a gold net, or a platinum net. The material 10 (C5) is positioned on a block of a good electric current conducting material that does not easily lead to redox processes 11 (C6). This material 11 typically consists of graphite, gold, or platinum. Liquid 1 (A1) then leaves the system and is discharged 12 (C7). Materials 8 and 11 (C3 and C6) are also separated by a suitable gaskets system 13 (C8) whose role is: (i) preventing escape of the liquid comprising the desired ionic species 1 (A1); (ii) helping to ensure that materials 8 and 11 (C3 and C6) are electrically insulated; and (iii) controlling the thickness and the area of the housing in which MAT and material 10 (C5) are to be placed, respectively.

The reference electrode 7 (C2) is connected to an external potentiostat/galvanostat 14 (D).

Materials 8 e 11 (C3 e C6) are contacted with the external potentiostat/galvanostat 14 (D) by means of electrodes 8a and 11a (C3a and C6a), respectively. If it is desired to control the flow of electric current flowing through MAT and/or the electrochemical potential applied to MAT, the external potentiostat/galvanostat 14 (D) carries out a “three-electrode” test on reactor 200 (C). In such a configuration, electrode 8a (C3a) and, consequently, material 8 (C3) and MAT act as a “working electrode”.

Materials 10 and 11 (C5, C6) and electrode 11a (C6a) act instead as a “counter electrode” finally, the reference electrode 7 (C2) acts as a “reference electrode” being put into ionic contact with MAT by the liquid comprising the desired ionic species 1 (A1) which is pumped through reactor 200 (C).

At the end of this process, the activated MAT is extracted from reactor 200 (C). Optionally, the activated material is further treated, for example by introducing it into vessel 2 (A2) and finally separating the activated MAT from liquid 1 (A1) (for example, by means of filtration, centrifugation and/or flotation).

According to the invention, STEP I and STEP II described above may be used by combining them for a desired number of times and in the desired order.

Preferably, the first and/or second steps are performed repeatedly 1 to 3 times.

In the present invention, it is possible to use any current or electrochemical potential control program by means of the potentiostat/galvanostat connected to it. Control protocols consisting of constant current or constant potential steps with a duration of 10 or more seconds are preferably applied.

Control protocols characterized by a “triangular wave” potential profile (also known as “cyclic voltammetr/) can also be used. It is preferable to avoid control protocols characterized by particularly fast dynamics (for example, pulsed) due to the high capacity typically shown by the samples of the material to be activated used and due to their high surface area. Such a capacity would “distort’ the potential actually applied on the material to be activated, thus reducing the precision in controlling the process parameters.

EXAMPLE 1

In a particular embodiment of the method, illustrated herein for illustrative and non limiting purposes, the following occurs:

• A1 is a 0.1 M aqueous solution of perchloric acid.

• A2 is a vessel with a volume equal to 1 L.

• A3 is 99.999% pure argon, which is bubbled through A1 for at least 15 min before starting the activation procedure.

• B1 is a peristaltic pump operating at a flow rate of 33 mL/min.

• C2 is a mercury/mercury sulfate/saturated potassium sulfate reference electrode.

• C3 is a block of graphite, perforated so that A1 gets on to MAT.

• MAT is a carbonitride-based electrocatalyst for the ORR, and having a core shell morphology, that is housed in reactor C from the beginning of the activation treatment. Since MAT is in the form of a powder, a porous layer of carbon fabric is inserted between MAT and C3 to prevent f MAT grains from being accidentally transported out of the system.

• C4 is a porous septum made of a layer of filter paper (thickness equal to 0.1 mm) stacked on a sheet of glass fiber with a thickness of 0.2 mm.

• C5 is made of carbon fabric. The geometric surface area of C5 is equal to 5 cm².

• C6 is a block of graphite, perforated so that A1 can pass through C5 and then being conveyed out of the reactor.

• C3a and C6a are copper plates coated with a 100 micrometers thick layer of gold. • Gaskets C8 are made of several layers of EPDM rubber; the thickness of the compartments in which MAT and C5 are housed is of about 0.8 mm; the surface area of these compartments is equal to 5 cm².

• D is a potentiostat/galvanostat.

In the embodiment described in this Example 1 , it is possible to activate a mass of carbonitride-based electrocatalyst for the ORR, and having a core-shell morphology, equal to about 100 mg per cm² of surface area of the housing whose size is determined by gaskets C8. Specifically, it is therefore possible to activate up to 450- 550 mg of electrocatalyst per batch.

EXAMPLE 2

This embodiment of the method, illustrated herein for illustrative and non-limiting purposes, is identical to that described in EXAMPLE 1 except for the following differences only: (i) the surface area of the housing, whose size is determined by gaskets C8, is equal to 50 cm²; (ii) the thickness of the housing, in which the powders of the carbonitride-based electrocatalyst for the ORR and having a core-shell morphology are placed, is 1.6 mm; and (iii) the flow rate of the 0.1 M perchloric acid aqueous solution is equal to 60 mL/min. Specifically, it is therefore possible to activate up to 9-11 g of electrocatalyst per batch.

EXAMPLE 3

This example describes the preparation of an EC-A by a preferred embodiment of the activation method according to the present invention.

In a first step, the EC-G indicated with the label “PtNU-G” is prepared using the procedure described in patent applications WO2017/055981 and WO2018/122368.

In short, the first stage of the preparation consists in the synthesis of the electrocatalyst “core”. This synthesis is carried out in accordance with the patent application WO2018/122368. 2,000 mg of graphene nanoplatelets are triturated together with 8,000 mg of ZnO nanoparticles having a diameter of approx. 50 nm for a duration of 15 hours. The resulting product is then treated for 1 h with an 5% HCI aqueous solution, and finally washed and dried. The resulting black powder is the electrocatalyst “core”.

The preparation of the “raw” electrocatalyst indicated with the label “PtNU-G” is carried out according to patent application WO2017/055981. In short, 450 mg of “core” obtained as described above are used to make the PtNi1-G precursor together with 450 mg of carbon black XC-72R, 338 mg of K₂PtCI₄, 2,796 mg of K₂Ni(CN)₄ and 450 mg of saccharose. The resulting precursor is subjected to a heat treatment under vacuum which is completed by a two-hour stage carried out at 900 °C. At the end of the process described, the “raw” electrocatalyst PtNi1-G is obtained. Such electrocatalysts comprises 26.5% Ni by weight (see TABLE 1); this value was determined by ICP-AES ( Inductively Coupled Plasma Atomic Emission Spectroscopy) analysis.

The activation process applied to PtNi1-G is carried out by adopting the setup described in EXAMPLE 1 . The procedure is articulated in the following steps.

1. A total of 415 mg of PtNi1-G are loaded in the setup. 110 mg of Ni are therefore contained in this electrocatalyst aliquot. Assuming that the main effect of the activation process is the removal of the nickel contained in PtNi1-G in the form of dissolved Ni²⁺ ions, the removal of all the Ni requires the flowing of a total charge of 361 C through the system.

2. The liquid containing the ionic species (in this case, a 0.1 M deaerated aqueous perchloric acid solution) is pumped through the reactor.

3. The flow of an oxidative current, whose intensity is shown in FIGURE 3(a), through the material is set up.

4. The ohmic resistance at the reactor terminals is measured by conventional PEIS ( Potentio Electrochemical Impedance Spectroscopy). This resistance is measured approximately every 30 C and is shown in FIGURE 3(b).

5. The terminal potential needed to make the current shown in FIGURE 3(a) flow through the system is monitored. This potential must be corrected by considering the ohmic drops introduced by the resistance at the terminals shown in FIGURE 3(b). The corrected potential value is shown in FIGURE 3(c). In order to prevent the electro-oxidation process of the platinum contained in the electrocatalyst, the corrected potential value should remain below 0.9 V vs. RHE. This process could in fact result in the formation of soluble platinum species that would be removed from the system, thus compromising the electrochemical functionality of the final activated material PtNi1-A.

6. The activation process is considered concluded when, even following the application of a “small’ oxidative current (20 mA) the potential at the terminals fails to remain below the set threshold of 0.9 V vs. RHE. In the present EXAMPLE 3, the activation process resulted to be concluded after flowing 364 C of charge through the system compared to the expected charge of 361 C to carry out the complete oxidation of the Ni present in the system to soluble Ni²⁺ species removed from the system. It has to be noted that, at the end of the activation process, 3.0% of Ni by weight is present within PtNi1-A. Thus, not all the charge passing through the system has served to remove the Ni. The excess charge probably served to electrolyze the water, as evidenced by the weak development of bubbles observed during the entire activation process.

7. At the end of the activation process, 275 mg of activated electrocatalyst PtNM- A are obtained.

The “activated” electrocatalyst PtNi1-A and the corresponding “raw” electrocatalyst PtNi1-G are subjected to chemical-physical characterization, as follows. Pt and Ni assays determined by ICP-AES are shown in TABLE 1.

FIGURE 4 shows the CV-TF-RRDE profiles of PtNi1-A in the ORR.

The results shown also include data relating to the reference electrocatalyst, indicated by the label “Pt/C”. This electrocatalyst, available on the market, has the typical features of a conventional state-of-the-art electrocatalyst. It comprises 10% by weight of platinum nanoparticles, which are supported on activated carbons.

The values shown in TABLE 1 clearly indicate that the activation treatment resulted in the removal of most of the nickel originally present in PtNi1-G. The analysis of the reactor wastewater, carried out using ICP-AES, revealed the presence of large amounts of nickel and an absolutely negligible concentration of platinum, of the order of a few ppb. Therefore, it is concluded that all the platinum originally present in the “raw” PtNi1-G remained in the activated electrocatalyst PtNi1-A. Based on this hypothesis, and knowing the percentage by weight of Pt and Ni in PtNi1-A, starting from 415 mg of PtNi1-G one would have expected to obtain 315 mg of PtNi1-A. Since only 275 mg of PtNi1-A were collected at the end of the process, the yield of the activation process was equal to 87.3%.

It is therefore noted that the activation process has completely changed the metal phases present in the system, expelling the nickel out and “driving” the PtNi1-G material towards the PtNi1-A system, characterized by a high activity in the ORR (see FIGURE 4).

Measurements shown in FIGURE 4 are obtained using a conventional experimental setup described in the literature [Adv. Fund Mater. 2007, 17, 3626-3638]. The overall Pt loading on the electrode is constant for the two materials and equal to 15 pg crrf². The ORR of the PtNi1-G electrocatalyst is not shown as it is extremely poor and unstable. ORR profiles of the two materials are very similar, both in terms of shape and diffusion limit current. The main difference is that PtNi1-A has an onset potential which is about 43 mV higher than the Pt/C reference electrocatalyst.

The electrochemical potential at which the faradic current density in the ORR is equal to 0.035 mA/cm², i.e. about 1/20 of the maximum diffusion current under the same experimental conditions (equal to 7 mA/cm²), is defined as the “onset potential’ .

For this reason, it can be concluded that PtNi-A is characterized by significantly higher performance in the ORR compared to the modern state-of-the-art.

EXAMPLE 4

This example describes the preparation of an EC-A by the method according to the present invention.

In a first step, the EC-G indicated with the label “PtCu1-G” is prepared using the procedure described in patent applications WO2017/055981 and WO2018/122368.

The preparation of the “raw” electrocatalyst indicated with the label “PtCu1-G” is carried out according to the patent application WO2017/055981 . In short, 900 mg of “core” obtained as described above are used to make the PtNi1-G precursor together with 900 mg of carbon black XC-72R, 338 mg of K₂PtCI₄, 281 mg of K₂Ni(CN)₄, 3.669 g Cu nanoparticle with a diameter of 25 nm and 900 mg of saccharose. The resulting precursor is subjected to a heat treatment under vacuum which is completed by a two-hour stage carried out at 900 °C. At the end of the process described, the “raw” electrocatalyst PtCu1-G is obtained. Such electrocatalysts comprises 62.1% by weight of Cu (see TABLE 2); this value was determined by ICP-AES analysis.

The activation process applied to PtCu1-G is carried out by adopting the setup described in EXAMPLE 1 . The procedure is articulated in the following steps.

1 . A total of 420 mg of PtCu1-G are loaded in the setup. 260 mg of Cu are therefore contained in this electrocatalyst aliquot. Assuming that the main effect of the activation process is the removal of the copper contained in PtCu1-G in the form of dissolved Cu⁺ ions, the removal of all the Cu requires the flowing of a total charge of 380 C through the system.

3. The flow an oxidative current, whose intensity is shown in FIGURE 5(a), through the material is set up. 4. The ohmic resistance at the reactor terminals is measured by conventional PEIS. This resistance is measured approximately every 30 C and is shown in

FIGURE 5(b).

5. The terminal potential needed to make the current shown in FIGURE 5(a) flow through the system is monitored. This potential must be corrected by considering the ohmic drops introduced by the resistance at the terminals shown in FIGURE 5(b). In order to prevent the electro-oxidation process of the platinum contained in the electrocatalyst, the corrected potential value should remain below 1.0 V vs. RHE. This process could in fact result in the formation of soluble platinum species that would be removed from the system, thus compromising the electrochemical functionality of the final activated material PtCu1-A.

6. The activation process is considered concluded when, even following the application of a “small’ oxidative current (20 mA) the potential at the terminals fails to remain below the set threshold of 1.0 V vs. RHE. In the present EXAMPLE 4, the activation process was concluded after flowing 458 C of charge through the system compared to the expected charge of 380 C to carry out the complete oxidation of the Cu present in the system to soluble Cu⁺ species removed from the system.

7. At the end of the activation process, 151 mg of activated electrocatalyst PtCu1-A are obtained.

The “activated” electrocatalyst PtCu1-A and the corresponding “raw” electrocatalyst PtCu1-G are then subjected to chemical-physical characterization, as follows. Pt and Ni assays determined by ICP-AES are shown in TABLE 2.

FIGURE 6 shows the CV-TF-RRDE profiles of PtCu1-A in the ORR. The results shown also include data relating to the reference electrocatalyst, indicated by the label “Pt/C".

The values shown in TABLE 2 clearly indicate that the activation treatment resulted in the removal of most of the nickel originally present in PtCu1-G. The analysis of the reactor wastewater, carried out using ICP-AES, revealed the presence of large amounts of copper and an absolutely negligible concentration of platinum, of the order of a few ppb. Therefore, it is concluded that all the platinum originally present in the “raw” PtCu1-G remained in the activated electrocatalyst PtCu1-A. Based on this hypothesis, and knowing the percentage by weight of Pt and Cu in PtCu1-A, starting from 420 mg of PtCu1-G one would have expected to obtain 157 mg of PtCu1-A. Since only 151 mg di PtNi1-A, were collected at the end of the process, the yield of the activation process was equal to 96.3%.

It is therefore noted that the activation process has completely changed the metal phases present in the system, expelling most of the copper out and “driving” the PtCu1-G materials towards the PtCu1-A system, characterized by a high activity in the ORR (see FIGURE 6).

Measurements shown in FIGURE 6 are obtained using a conventional experimental setup described in the literature [Adv. Fund Mater. 2007, 17, 3626-3638]. The overall Pt loading on the electrode is constant for the two materials and equal to 15 pg crn ². The ORR of the PtCu1-G electrocatalyst is not shown as it is extremely poor and unstable. PtCu1-A has an onset potential which is about 41 mV higher than the Pt/C reference electrocatalyst. For this reason, it can be concluded that PtNi-A is characterized by significantly higher performance in the ORR compared to the modern state-of-the-art.

EXAMPLE 5

This example describes the preparation of two EC-As by the method according to the present invention.

In a first step, the EC-G indicated with the label “PtCu2-G” and “PtCu3-G” are prepared using the procedure described in patent applications WO2017/055981 and WO2018/122368.

The activation process applied to PtCu2-G and PtCu3-G is carried out by adopting the experimental setup described in FIGURE 2 with a procedure roughly indicated as

STEP 1.

The vessel A2 is filled with 500 mL of a sulfuric acid aqueous solution having a concentration equal to 9.2 mol/L (liquid comprising the desired ionic species A1). 500 mg of PtCu2-G are introduced in A1 ; the resulting suspension is placed under vigorous stirring and thermostated at 75 °C. This activation treatment lasts for 48 hours. At the end of this treatment, the suspension is filtered and washed with bidistilled water. The resulting product is the activated electrocatalyst indicated as “PtCu2-G”. The activated electrocatalyst indicated as “PtCu3-A” is prepared starting from the raw electrocatalyst PtCu3-G following the same procedure described above for the activated electrocatalyst PtCu2-A. Pt, Cu and Ni assays of PtCu2-A and PtCu3-A are shown in TABLE 3.

The measurements shown in FIGURE 7 and FIGURE 8 are obtained using a conventional experimental setup described in the literature [Adv. Funct. Mater. 2007, 17, 3626-3638\. All voltammograms were recorded after the conditioning step of the catalytic layer described in [Journal of The Electrochemical Society, 162 (10) F1144- F1158 (2015)]. The overall Pt loading on the electrode is constant for the two materials and equal to 15 pg crn ². It has to be noted that this EXAMPLE 5 uses also a different electrocatalyst electrode than the one used in EXAMPLE 3 and EXAMPLE 4. The electrocatalyst electrode EXAMPLE 5 is a Pt/C electrocatalyst electrode which has a platinum filling equal to 40%wt, indicated as “Pt/C ref. 2”. This choice was made to compare electrocatalysts (Pt/C ref. 2 and PtCu3-A) having a platinum loading as similar as possible to each other (PtCu3-A: 29.5 %wt Pt; Pt/C ref. 2: 40%wt Pt). The electrocatalyst PtCu2-A was instead compared with the electrocatalyst Pt/C ref. since these two materials have a very similar platinum filling (PtCu2-A: 7.92 %wt Pt; Pt/C ref.: 10%wt Pt).

PtCu2-A has an onset potential higher than the Pt/C reference electrocatalyst Pt/C ref. of a value equal to 34 mV (PtCu2-A). PtCu3-A has instead an onset potential higher than the Pt/C reference electrocatalyst Pt/C ref. 2 of a value equal to 35 mV. For this reason, it can be concluded that PtCu2-A and PtCu3-A are characterized by significantly higher performance in the ORR compared to the modern state-of-the-art.

EXAMPLE 6

TABLE 4 shows the onset potential of “activated” electrocatalysts obtained by the setup and procedures according to the invention as compared to the reference electrocatalysts.

Claims

1 . A method of activating an initial solid catalyst material comprising a first step and a second step in sequence, each step comprising treating said solid material with a liquid containing ionic species, wherein an electric current and/or an electrochemical potential is applied in said first or second step, and at least one subsequent separation step of separating the activated solid catalyst material from the liquid.

2. Method according to claim 1 , characterized in that said first and/or second steps are carried out at least once, preferably carried out repeatedly 1 to 3 times, and optionally a separation step of separating the activated solid material from the liquid is carried out downstream of said first and/or second steps.

3. Method according to claim 1 or 2, characterized in that said solid material is in the form of powders.

4. Method according to any one of the preceding claims, characterized in that said solid catalyst material is an electrocatalyst, preferably a carbon itride-based electrocatalyst having a core-shell morphology, a catalyst based on metallic elements of the platinum group, preferably palladium, platinum and/or rhodium, or a catalyst comprising graphene oxide, graphene nitride, graphene, or graphene functionalized with -COOH and/or -OH groups.

5. Method according to any one of the preceding claims, characterized in that said liquid containing ionic species is a solvent selected from the group comprising water, alcohols, organic acids, organic carbonates, ionic liquids, and a mixture thereof.

6. Method according to claim 5, characterized in that said liquid containing ionic species is a solution comprising (i) at least one soluble ionic compound, preferably selected from NaCI, KBr, NaF, Csl, NaN0₃, K₂S0₄, or a mixture thereof, (ii) at least one acid, preferably selected from HCI, HN0₃, HCI0₄, H₂S0₄, H₃P0₄, CH₃COOH, or a mixture thereof, (iii) or at least one NaOH, KOH, Mg (OH)₂, NH₃ base, or a mixture thereof.

7. Method according to claim 6, characterized in that said solution has a pH from -2 to 16, preferably from 0 to 3 or 11 to 14.

8. Method according to any one of the preceding claims, characterized in that in said first and/or second steps a gas, preferably a gas selected from nitrogen, argon, neon, helium, krypton, xenon, oxygen, hydrogen, carbon dioxide, or a mixture thereof, is bubbled through said liquid.

9. Method according to any one of the preceding claims, characterized in that said first and/or second steps are carried out by means of stirring and/or heating.

10. Method according to any one of the preceding claims, characterized in that said first and/or second steps are carried out at a temperature comprised between - 200 °C and 500 °C for a time comprised between 0.01 and 600 hours, preferably between about 20 °C and 75 °C for a time comprised between about 0.1 and 48 hours.

11. Method according to any one of the preceding claims, characterized in that said electrochemical potential is lower than 1 .5 V vs. RHE, preferably between 0.2 and 0.9 V vs. RHE.

12. Method according to any one of the preceding claims, characterized in that said electric current is comprised between 0.00001 and 5000 mA, preferably 50 mA.

13. Method according to any one of the preceding claims, characterized in that said first step is carried out in a first vessel and said second step is carried out in a second vessel, wherein the liquid containing ionic species is transferred from the first to the second vessel, or vice versa.

14. Method according to any one of the preceding claims, characterized in that the step of applying an electric current and/or an electrochemical potential comprises a) a first sub-step of entering the liquid into a first vessel containing a reference electrode and b) a second sub-step of transferring the liquid from said first vessel to a second vessel containing a working electrode.

15. Method according to any one of the preceding claims, characterized in that said separation step of separating the activated solid material from the liquid is obtained by means of filtration, centrifugation and/or flotation.

16. An activated solid catalyst material obtainable by the method of activating an initial solid catalyst material according to any one of the preceding claims, said initial solid material being preferably an electrocatalyst, more preferably a carbonitride- based electrocatalyst having a core-shell morphology, a catalyst based on metallic elements of the platinum group, more preferably palladium, platinum and/or rhodium, or a catalyst comprising graphene oxide, graphene nitride, graphene, or graphene functionalized with -COOH and/or -OH groups.

17. A fuel cell, an electrolyzer, a metal-air battery, or a catalytic converter containing an activated solid catalyst material according to claim 16.

18. A plant for carrying out the method of activating an initial solid catalyst material according to claim 1 , comprising a first apparatus and a second apparatus containing a liquid including ionic species and sequentially connected for transferring the liquid from the first to the second apparatus or vice versa, wherein a working electrode is present in said first or second apparatus.

19. A plant according to claim 18, wherein the apparatus containing the working electrode is an electrochemical reactor, said reactor preferably comprising a first vessel containing a reference electrode and a second vessel for transferring the liquid, said second vessel containing a working electrode.